1. Field of the Invention
This invention relates to an apparatus for detecting information relating to the relative displacement of an object to be measured, such as the amount of displacement, the velocity or the like. The invention also relates to a driving system which uses the information detecting apparatus.
In particular, the invention can be reliably applied to an optical displacement sensor, in which light is projected onto an object, and a physical parameter, such as the displacement or velocity, associated with the object is detected using optical means. The invention may also be applied to a driving system which uses the optical displacement sensor.
2. Description of the Related Art
Among optical displacement sensors of this kind, for example, optical encoders, laser interferometers, and the like have been widely used in the fields of NC (numerical control) machine tools, OA (office automation) apparatuses, robots, precision manufacturing apparatuses, and the like.
FIG. 1 is a diagram illustrating the configuration of an optical displacement sensor developed by the assignee of the present application and described in U.S. Pat. No. 5,283,434.
In FIG. 1, light emitted from a light-emitting device 101, such as a semiconductor laser, a light-emitting diode, or the like, is converted into a light beam by a lens 102, and is incident upon a diffraction grating 103A. The light beam is subjected to transmission diffraction by the diffraction grating 103A, and is divided into a plurality of light beams including a 0-order diffracted light beam R.sub.0, a +1st-order diffracted light beam R.sub.+1, and a-1st-order diffracted light beam R.sub.-1, which beams are incident upon a diffraction grating 112 formed on a scale 111. The diffraction grating 103A, diffraction gratings 103B1, 103B2 and 103C1, 103C2, and the diffraction grating 112 on the scale 111 have the same pitch (for example, 1.6 .mu.m).
The directly-advancing 0-order diffracted light beam R.sub.0 is subjected to reflection diffraction at a point P1 on the diffraction grating 112, and is divided, for example, into a +1st-order diffracted light beam R.sub.0.sup.+1, and a -1st-order diffracted light beam R.sub.0.sup.-1, which beams are subjected to phase modulation. When the scale 111 has moved relative to the diffraction gratings 103, the phase of the +1st-order diffracted light beam R.sub.0.sup.+1 is shifted by +2.pi.x/P, and the phase of the +1st-order diffracted light beam R.sub.0.sup.-1 is shifted by -2.pi.x/P, where x is the amount of movement of the diffraction grating 112, and P is the pitch of the diffraction grating 112.
The above-described +1st-order diffracted light beam R.sub.0.sup.+1 is again subjected to transmission diffraction by diffraction gratings 103B1 and 103B2, and is divided into a 0-order diffracted light beam R.sub.0.sup.+1.sub.0, a -1st-order diffracted light beam R.sub.0.sup.+1.sub.-1, and the like. The -1st-order diffracted light beam R.sub.0.sup.+1.sub.-1 becomes perpendicular to the surface of the diffraction grating, and the phase of the beam's wavefront equals +2.pi.x/P.
The -1st-order diffracted light beam R.sub.0.sup.-1 is subjected to transmission diffraction by diffraction gratings 103C1 and 103C2, and is divided into a 0-order diffracted light beam R.sub.0.sup.-1.sub.0, a +1st-order diffracted light beam R.sub.0.sup.-1.sub.+1, and the like. The +1st-order diffrated light beam R.sub.0.sup.-1.sub.+1 becomes perpendicular to the surface of the diffraction grating, and the phase of the beam's wavefront equals -2.pi.x/P.
If the phase of the grating's arrangement of the diffraction grating 103C1 is shifted by P/4 relative to that of the diffraction grating 103B1, the phase of the wavefront of the +1st-order diffracted light beam R.sub.0.sup.-1.sub.+1 is shifted further by -2.pi.(P/4)/P=-.pi./2 to become -2.pi.x/P-.pi./2. If the phase of the grating's arrangement of the diffraction gratings 103B2 and 103C2 are shifted by P/2 relative to those of the diffraction gratings 103B1 and 103C1, respectively, then the phases of respective wavefronts have the following values:
______________________________________ 103B1: -2.pi.x/P 103B2: -2.pi.x/P - .pi. 103C1: -2.pi.x/P - .pi./2 103C2: -2.pi.x/P - 3.pi./2. ______________________________________
The light beam R.sub.+1 subjected to +1st-order diffraction by the diffraction grating 103A is subjected to reflection diffraction at a point P2 on the diffraction grating 112 on the scale 111, and is divided into a -1st-order diffracted light beam R.sub.+1.sup.-1, a 0-order diffracted light beam R.sub.+1.sup.0, and the like. The phases of the respective light beams are modulated.
The -1st-order diffracted light beam R.sub.+1.sup.-1 is incident upon the diffraction gratings 103B1 and 103B2 while the beam's phase is shifted by -2.pi.x/P. The phase of the wavefront of the directly-advancing 0-order diffracted light beam R.sub.+1.sup.-1.sub.0 is -2.pi.x/P.
The light beam R.sub.-1 subjected to -1st-order diffraction by the diffraction grating 103A is subjected to reflection diffraction at a point P3 on the diffraction grating 112 on the scale 111, and is divided into a +1st-order diffracted light beam R.sub.-1.sup.+1, a 0-order diffracted light beam R.sub.-1.sup.0, and the like. The phases of the respective light beams are modulated. The+1st-order diffracted light beam R.sub.-1.sup.+1 is incident upon the diffraction gratings 103C1 and 103C2 while the beam's phase is shifted by +2.pi.x/P. The phase of the wavefront of the directly-advancing 0-order diffracted light beam R.sub.-1.sup.+1.sub.0 equals+2.pi.x/P.
The light beams R.sub.+1.sup.-1.sub.0 and R.sub.0.sup.+1.sub.-1 whose optical paths are superposed at the diffraction gratings 103B1 and 103B2 become interfering light, which is incident upon photosensors 104B1 and 104B2. At that time, the interference phases of the light beams incident upon the photosensors 104B1 and 104B2 are:
(+2.pi.x/P)-(-2.pi.x/P)=4.pi.x/P, and PA1 (-2.pi.x/P-.pi.)-(+2.pi.x/P)=-4.pi.x/P-.pi., respectively. PA1 (-2.pi.x/P-.pi./2)-(+2.pi.x/P)=-4.pi.x/P-.pi./2, and PA1 (-2.pi.x/P-3.pi./2)-(+2.pi.x/P)=-4.pi.x/P-3.pi./2, respectively Every time the diffraction grating 112 on the scale 111 moves by 1/2 pitch, a light-and-dark signal of one period is generated. The pitches of a light-and-dark pattern are shifted by the 1/4 period from each other in the photosensors 104B1 and 104B2.
Every time the diffraction grating 112 on the scale 111 moves by 1/2 pitch, a light-and-dark signal of one period is generated. A signal Whose phase is inverted from that of a signal obtained from the photosensor 104B1 can be obtained from the photosensor 104B2. If the pitch of the diffraction grating equals 1.6 .mu.m, a sinusoidal signal having a period of 0.8 .mu.m is obtained.
The light beams R.sub.-1.sup.+1.sub.0 and R.sub.0.sup.-1.sub.+1 whose optical paths are superposed at the diffraction gratings 103C1 and 103C2 become interfering light, which is incident upon photosensors 104C1 and 104C2. At that time, the interference phases of the light beams incident upon the photosensors 104C1 and 104C2 are:
That is, encoder signals, serving as periodic signals having periods shifted by the 1/4 period from each other, which are usually called an A-phase signal and a B-phase signal, whose periods change in accordance with the relative movement of the diffraction grating 112 on the scale 111, can be obtained at terminals A and B through differential circuits 105B and 105C, respectively, each of which provides the difference between the to signals.
By using the A-phase signal and the B-phase signal, the amount and the direction of relative movement of the diffraction grating 112 on the scale 111 can be measured by a well-known method.
As described above, in accordance with the displacement of the scale 111, periodic signals having periods shifted by the 1/4 period from each other are obtained from the photosensors 104B1, 104B2, 104C1 and 104C2. A state of relative displacement between the above-described sensor unit and the scale 111 can be detected based on the obtained signals using a known signal processing circuit (not shown).
In accordance with the distance between the scale 111, and the photosensors 104B1, 104B2, 104C1 and 104C2, and the state (relating to divergence and convergence) of the light beams, lenses may be omitted or added.
The diffraction gratings 103B1, 103B2, 103C1, and 103C2 are usually provided on a single transparent plate (for example, a glass substrate) using a replica technique or photoetching. In the case shown in FIG. 1, the portion (the sensor unit) other than the scale 111, which includes the diffraction grating 112, is accommodated within a single case, and is usually called a "head".